Control of a living radical polymerization of methacrylates by light.

The ability to precisely control molecular weight and molecular weight distributions, as well as gain sequence and architecture control in polymer synthesis is of considerable importance and has greatly impacted the advancement of science and technology. Indeed, the development of controlled living polymerization methods has profoundly changed polymer research with strategies, such as nitroxidemediated radical polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer polymerization (RAFT), allowing the facile synthesis of well-defined polymers that are diverse in both their structure and function. Recently there has been an effort to dramatically increase the scope of living radical polymerization through the development of strategies to regulate the activation and deactivation steps by using an external stimulus. Arguably, the most successful strategy that controls both the initiation and growth steps has been the recent work of Matyjaszewski and co-workers who exploited the unique aspects of electrochemistry to control the ratio of activator to deactivator in ATRP. By selective targeting of redox-active catalytic species, the polymerization reaction could be turned “on” and “off” by adjusting parameters such as applied current, potential, and total charge passed. As with traditional radical polymerization, the most robust and widely used form of regulation is through photopolymerization, which is a pervasive procedure in both academia and industry. The ability to develop a photocontrolled living radical polymerization would, therefore, represent a significant breakthrough. Interestingly, one of the earliest attempts to develop a living radical polymerization involved iniferter polymerization using a dithiocarbamate under UV irradiation. However, the procedure was intrinsically limited and poor control and broad molecular weight distributions were obtained. Subsequently, photoinitiation of ATRP, NMP, and RAFT polymerizations have been developed, though in all cases only the initiation step was photocontrolled and all subsequent growth steps could not be photoregulated. As a result, the development of a highly responsive photocontrolled living radical procedure, which affords control over the chain growth process, is both a major opportunity as well as challenge for the future of living polymerizations. The key to addressing this challenge was recent work by the research groups of Macmillan, Yoon, Stephenson, and others who have exploited the power of photoredox catalysts for organic transformations that are mediated by visible light. We envisaged that the unique properties of these photoredox catalysts would allow for the development of a highly responsive photocontrolled living radical polymerization. Our proposed mechanism for this process is shown in Scheme 1. The fac-[Ir(ppy)3] (1, Figure 1), a com-